1. Field of the Invention
The present invention relates to a semiconductor laser, and in particular to a semiconductor laser used as the light source for an optical disk system, for information processing, or for optical communications.
2. Description of Related Art
Conventional semiconductor lasers employed as the light source in optical disk systems have been manufactured, for example, by the following process. First, a first crystal growth step is conducted to form at least a first conduction-type cladding layer, an active layer, and a second conduction-type cladding layer in that order on a first conduction-type semiconductor substrate. Then a stripe formation process for forming a region where current is introduced to the active layer is performed, and then epitaxial growth to bury the stripe portion is performed.
For example, the process for manufacturing a ridge-type semiconductor laser element like that shown in FIGS. 9A to 9E generally includes three crystal growth steps (formation of the double heterostructure, formation of the current blocking layer, and formation of the burying layer). First, as shown in FIG. 9A, an n-type cladding layer 2, an active layer 3, and a p-type cladding layer 4 are deposited and grown on an n-type substrate 1 in that order by a first crystal growth step. Then, as shown in FIG. 9B, the p-type cladding layer 4 is etched to form a ridge portion 4a by a process employing photolithography. Next, as shown in FIG. 9C, an n-type current blocking layer 8 is formed through a second crystal growth step. Then, as shown in FIG. 9D, a p-type burying layer 9 is formed through a third crystal growth step. Lastly, as shown in FIG. 9E, a p-side ohmic contact electrode 13 is formed on the p-type burying layer 9 and an n-side ohmic contact electrode 12 is formed on the bottom surface of the n-type substrate 1.
The groove-type semiconductor laser element shown in FIGS. 10A to 10D also requires two crystal growth steps (formation of the double heterostructure and formation of the burying layer). First, as shown in FIG. 10A, an n-type cladding layer 2, an active layer 3, a p-type cladding layer 4, and an n-type current blocking layer 8 are deposited and grown in that order on an n-type substrate 1 through a first crystal growth step. Next, as shown in FIG. 10B, the n-type current blocking layer 8 is etched to form a groove portion 8a by a process employing photolithography. Then, as shown in FIG. 10C, a p-type burying layer 9 is formed in a second crystal growth step. Lastly, as shown in FIG. 10D, a p-side ohmic contact electrode 13 and an n-side ohmic contact 12 are formed.
This plurality of crystal formation steps was a major hurdle in reducing the manufacturing costs for laser chips. Accordingly, as a method for omitting the burying crystal growth step so as to fabricate a semiconductor laser element through a single crystal growth step, an element that has a ridge-type waveguide structure and that constricts the current and confines light by a dielectric film made of SiO2 or Si3N4 or the like has been developed and produced (for example, see J. Hashimoto et. al., IEEE J. Quantum Electron, vol. 33, pp. 66-70, 1997). An example of this method is shown in FIGS. 11A to 11D. First, as shown in FIG. 11A, in a first crystal growth step, an n-type cladding layer 2, an active layer 3, and a p-type cladding layer 4 are deposited and grown in that order on an n-type substrate 1 so as to form a double heterostructure. Next, as shown in FIG. 11B, the p-type cladding layer 4 is etched to form a ridge portion 4a by a process employing photolithography. Then, as shown in FIG. 1C, an insulating film (dielectric film) 11 is formed and then etched using a photoresist mask (not shown) to expose the p-type cladding layer 4. Lastly, as shown in FIG. 1D, a p-side ohmic contact electrode 13 and an n-side ohmic contact electrode 12 are formed.
However, with the element shown in FIG. 11D, the unevenness formed in the element surface is affected by the ridge portion and becomes large because a burying crystal growth step is not performed after the stripe is formed. When the electrode side surface having this uneven surface is adopted as the bonding surface during assembly of the semiconductor laser, there is the problem that stress during chip bonding tends to concentrate in the ridge portion 4a, thereby deteriorating the properties of the semiconductor laser.
In a presentation by Miyashita et. al. at the Japan Society of Applied Physics Annual Meeting in the spring of 2000 (presentation No. 29a-N-7, “Effective Refractive Index Type High-Output Low-Operation-Current Laser for DVD-RAM”), it was reported that by also forming protruding portions to the left and right of the ridge portion, it is possible to reduce damage during bonding, and that a significant improvement in laser properties can be achieved by adopting such a structure for a high output red semiconductor laser element. However, the height difference between the ridge portion and the other protruding portions was the thickness of the SiO2 film employed for current confinement (approximately 0.1 μm).
Also, JP H12-164986A discloses a method to keep stress from concentrating at the ridge portion during assembly by using regrowth to form protruding portions that are higher than and sandwich the ridge portion. This example is shown in FIGS. 12A to 12D. First, as shown in FIG. 12A, an n-type cladding layer 2, and active layer 3, and a p-type cladding layer 4 are deposited and grown in that order on an n-type substrate 1 through a first crystal growth step, and furthermore an oxidation prevention layer 10 is laminated. Next, as shown in FIG. 12B, a protective film 14 serving as an insulation layer is deposited, and photolithography is used to form a resist mask to remove the outside regions of the protective film 14, after which an n-type current blocking layer 8 is formed in those regions in a first selective growth step. Then, as shown in FIG. 12C, photolithography is used to provide an aperture portion for ridge formation in the center portion of the protective film 14, and a p-type second cladding layer 15 and a p-type contact layer 16 are deposited and grown in that order on this aperture portion and on the n-type current blocking layers 8 on both sides in a second selective growth step. Finally, as shown in FIG. 12D, a p-side ohmic contact electrode 13 and an n-side ohmic contact electrode 12 are formed. However, the selective growth steps required by this manufacturing method make it difficult to reduce the cost of the chip.
Moreover, JP H11-25 1679A discloses the use of the structure shown in FIG. 13 as a method for reducing damage due to unevenness during bonding in a ridge-type semiconductor laser for which a burying growth step is performed. That is, a second p-side ohmic contact electrode 14 is provided to make the thickness of the electrode on the uneven surface side thick at the recessed portion and thin at the protruding portion, so that the unevenness in the semiconductor surface is reduced. It should be noted that a protruding portion is also unsatisfactory in conventional ridge-type semiconductor lasers for which a burying growth step is performed.
The following four points are the main problems for the semiconductor laser element fabricated by a single crystal growth step and shown in FIGS. 11A to 11D.
The first problem is the deterioration of laser properties due to damage caused during bonding, as mentioned above. With the structure proposed by Miyashita et. al., the difference in height between the ridge portion and the other protruding portions is too small, and thus damage during bonding cannot be reduced sufficiently and there is a high risk that large stress will be applied to the ridge portion as well. Furthermore, with the procedure mentioned in JP H11-251679A, a difference of about several m in the electrode film thickness must be formed at the chip surface, the increase in thickness of the electrode film leads to larger discrepancies in the electrode film thickness, and so mass-productivity drops for devices that are assembled with the uneven surface as the reference surface.
The second problem is that a semiconductor laser element with this structure has a larger thermal resistance than conventionally structured elements. This is because the dielectric film made of SiO2 or Si3N4, for example, has a considerably lower thermal conductivity than the semiconductor film, and thus a semiconductor laser element with the majority of its surface covered by a dielectric film has inferior heat dissipation properties as compared with conventionally structured elements. This leads to concerns regarding the deterioration of laser properties, especially at elevated temperatures, and a drop in reliability.
The third problem is that damage at a ridge portion due to the concentration of stress at the ridge portion and production defects such as cracks occur more easily during the cleavage process in the direction perpendicular to the stripe direction, which is performed for the purpose of forming in the chip end surface the mirror necessary to form a Fabry-Perot resonator. It is thought that the primary reason why stress tends to concentrate at the ridge portion is that there is a larger difference in height at the protruding portions than in a conventionally structured element because burying growth is not performed in the former. On the other hand, if the structure proposed by Miyashita et. al. or the method mentioned in JP H12-164986A is adopted, then a plurality of protruding portions are formed in the element surface, and thus it is conceivable that the stress concentrating at the ridge portion will be reduced. However, simply having a plurality of protruding portions is not enough, and no effect can be anticipated if the other protruding portions are spaced considerably from the ridge portion. There is no mention by Miyashita et. al. of the spacing between the ridge portion and the other protruding portions. Also, with the structure mentioned in JP H12-164986A, regrowth of the ridge portion is performed with the insulation film serving as a mask, and thus the insulation film is not formed at the ridge portion slope. Consequently, stress is easily concentrated at the boundary portion between the ridge bottom portion and the insulation film, and the regrowth interface is close, so that crystallinity differences near the ridge bottom portion tend to cause damage such as cracking to the ridge bottom portion.
The fourth problem is a reduction in the contact area with the electrode as a result of the burying growth step not being performed, resulting in an increase in element resistance. The increase in resistance leads to diminished frequency properties for the element and more power consumption, among others. JP H12-164986A discloses a method for forming an electrode at the ridge portion slope in order to increase the contact area and reduce the resistance. However, in general, the ridge portion is designed to have a composition with high concentration of Al, and a semiconductor layer surface with a high Al composition oxidizes quickly, frequently leading to the problem of electrode peeling.